In general, in order to use the so-called three-way catalytic converter for simultaneously treating HC, CO and NO.sub.x that are noxious contents in the exhaust gas in an internal combustion engine, it is necessary to control the air-fuel ratio of the engine to a given value with high accuracy throughout all of the operating conditions of the engine. Consequently, with the air-fuel ratio control system in an internal combustion engine using a conventional carburetor, air-fuel ratio control is effected as follows. The air-fuel ratio control is effected by the carburetor during normal operating condition, and fuel is fed to the air intake system by use of a mechanical accelerator pump or negative pressure detecting and operating accelerator pump during acceleration. The air-fuel ratio is maintained within a given range by controlling the open-close speed of the throttle valve or the amount of secondary air fed to the air-intake system during deceleration.
With the air-fuel ratio control system in an internal combustion engine using a conventional carburetor of the type described above, for example, an auxiliary amount of fuel is adapted to be fed to the air intake system by use of the mechanical accelerator pump, negative pressure detecting and operating accelerator pump or the like during acceleration. However, the response is poor and the required supplementary amount of fuel corresponding to the change in the operating condition of the engine cannot be supplied. Hence, the fluctuation in the air-fuel ratio is high, resulting in a temporary lean air-fuel ratio. On the other hand, the air-fuel ratio is controlled by controlling the open-close speed of the throttle valve or the amount of secondary air fed to the air intake system during deceleration. The response here is also poor, and hence the fluctuation in the air-fuel ratio is high in the same manner as during acceleration, resulting in a temporary rich air-fuel ratio. Consequently, in the case of an exhaust gas purifying system using the three-way catalytic converter of the type described above, there has been encountered disadvantages in that it is very difficult to simultaneously treat HC, CO and NO.sub.x at a high purification rate because the carburetor cannot satisfactorily feed-back control to the theoretical air-fuel ratio.
As another type of conventional air-fuel ratio control system in internal combustion engines for motor cars, there have been used the so-called electronically controlled fuel injection system. In this, fuel injection valves are provided on respective cylinders of the engine and fuel is fed to the air intake system of the internal combustion engine by electrically or mechanically opening and closing the fuel injection valves for control without the use of a carburetor. Even in an engine with such an electronically controlled fuel injection system, the air-fuel ratio of the engine is sometimes shifted from the controlled air-fuel ratio due to delay in the air intake system and fuel system during acceleration or deceleration of the engine. Namely, during sharp acceleration of the engine, negative pressure in an air intake manifold sharply increases to atmospheric pressure, and hence, only part of the fuel injected into the air intake manifold is evaporated. This causes the amount of fuel sucked into the cylinders of the engine to decrease, resulting in a lean air-fuel ratio. On the other hand, during sharp deceleration of the engine, pressure in the air intake manifold decreases to almost vacuum pressure, whereby liquid fuel in the air intake manifold is evaporated in a large amount. This results in a rich air-fuel ratio.
In such an engine with an electrically controlled fuel injection system, in order to cope with the delay in both the air intake system and the fuel system, for example, the accelerating or decelerating condition of the engine is detected by differentiating the air intake pressure or the opening degree of the throttle. When the value of differentiation is higher than a given value, i.e. the transient variation value becomes larger to a certain extent, fuel is increased or decreased commensurate to the transient valuation value so as to correct the air-fuel ratio. Alternatively, until the transient variation value reaches a peak value during acceleration or deceleration of the engine, the value commensurate with the detected transient variation value is regarded as the correction value, and after the transient variation value reaches the peak value, a value obtained by decreasing the peak value at a certain time constant is regarded as the correction value, thus enabling correction of the temporary variations in air-fuel ratio during acceleration or deceleration.
Consequently, in an internal combustion engine with such an electronically controlled fuel injection system, the dispersion in the standing characteristics due to the tolerance in manufacture and problems of response during acceleration or deceleration in the use of the carburetor can be obviated. However, the necessity of providing fuel injection valves on respective cylinders leads to very high cost. Moreover, the dispersion between the respective fuel injection cylinders causes problems. Additionally, since liquid fuel is directly injected into the air intake system by means of the fuel injection valve, there is encountered a problem in that the atomization of fuel is not necessarily and satisfactorily effected.
In order to avoid the high cost involved with the above-mentioned electronically controlled fuel injection unit, there has been proposed such a system wherein a single fuel injection valve of large capacity is provided relatively upstream of the air intake system in place of the carburetor, so that the fuel injection flowrate can be electrically controlled. In this case, the flowrate of fuel which should be controlled by the single fuel injection valve becomes very large, thus causing a problem of difficulty in delicate control.